This book offers a comprehensive survey of the agreement phenomena found in written and spoken Arabic. It focuses on both the synchronic description of these agreement systems, and the diachronic question of how they evolved. To answer these questions, large amounts of data have been collected and analysed, ranging from 6th century poetry and Quranic Arabic to the contemporary dialects. The results presented by the authors of this research greatly improve our understanding of Arabic syntax, and challenge some well-established views. Can Arabic be envisioned as possessing more than only two genders? Are some contemporary dialects more similar to the pre-Classical version of the language than MSA is? And is the Standard rule prescribing feminine singular agreement with nonhuman plurals a more recent development than previously thought?